The present invention relates to a device for the continuous injection of a mixture consisting of fuel and air into the cylinders of an internal combustion engine. A fuel injection system of this type, in which a mixture of both air and fuel are supplied by the fuel delivery pump (rather than liquid fuel only) is described in commonly ownded U.S. Pat. application Ser. No. 932,493, filed Aug. 10, 1978 now U.S. Pat. No. 4,368,714. As described therein, a venturi is arranged in the air intake pipe upstream of the throttle flap. An air channel branches off from the narrowest point of the venturi to supply the needed air for the fuel delivery pump. A fuel metering device supplies fuel into the mixing chamber portion of this channel substantially as a function of existing pressure difference between the instantaneous pressure at the venturi and atmosphere, or in other words, the rate of air flow through the channel (since with increasing air flow rate the suction created on the fuel metering output line increases). A fuel delivery pump is connected to receive the air and fuel existing from the channel and to supply the mixture to the points of injection associated with the several cylinders of the engine. This particular type of injection system is advantageous since the fuel is mixed with a portion of the air flow in the intake pipe prior to the injection into the main air flow. This results not only in a more effective preparation of the fuel, since it is more thoroughly atomized, but also obviates the difficulties of accurate proportioning and adequate dispersion which normally occur in straight liquid injection (where the liquid fuel is dispersed through an injection nozzle), especially at low rates of injection.
An injection system of this type, in addition to the main fuel proportioning apparatus which supplies fuel as a function of load between partial and full load, generally has a separate idling fuel metering system, operative in idling only. Difficulties can arise in such injection systems, however, especially in the so-called transition range, that is a transition from the idling load, wherein fuel is supplied from the idling metering system, to partial full load when fuel is supplied from the main fuel metering system. To achieve a smooth engine performance during transition, generally an additional fuel enrichment is required.